PROMISES FROM MTN BRING NO RESULT

I was called with an offer for a cellphone contract, received the phone in the mail and took it to the local branch for a sim swop. I was informed that the package does not allow a swop. I subsequently arranged a cancellation of this contract and took out a new one. The phone was sent back. All of this occured in May. I have since been billed monthly for a contract I do not have. Calls to MTN are all unsuccessful. I have spent hours on calls to MTN and WAS made to believe that the matter WAS resolved, but month after month I am billed once more.